Is your goal a or a modernized version with SD card support?
The ULA in the ZX Spectrum performed several critical roles simultaneously:
The serves as the ultimate masterclass in minimalist retro engineering, proving that a fully functional microcomputer can be designed using a single custom chip to handle video, audio, and memory management. Is your goal a or a modernized version with SD card support
Managing access to the lower 16KB of RAM between the CPU and the video display generator.
If using a 3.3V FPGA with an original 5V Z80 CPU, use bidirectional level shifters (like the 74LVC4245A) to protect your logic gates. If using a 3
can emulate the Z80 CPU, RAM, and ULA simultaneously inside a single chip, making a portable layout incredibly simple. 3. Solving Memory Contention
: The ULA reads 40 bytes of screen data per line (6.5 µs fetch) then the Z80 runs (6.5 µs). This “contended memory” model is the single most difficult feature to emulate accurately. Solving Memory Contention : The ULA reads 40
Here is a step-by-step design guide for a minimal Z80 system:
+-------------------------------------------------------------+ | ZX SPECTRUM | | | | +-----------+ +-----------+ +---------+ | | | | Buses | | Buses | | | | | Z80 CPU |<=======>| ULA |<=======>| RAM | | | | | | (Custom | | | | | +-----------+ | Silicon) | +---------+ | | +-----+-----+ | | | | | v | | Video / Audio | +-------------------------------------------------------------+
If you are looking to build a portable retro machine today, follow these core design pillars: 1. The Brain (The CPU) Authentic: Use a real